Make constraints tests more robust

I don't think checking the result of repr is sufficient for many of the tests in this file. In particular those where the repr just says a lambda or function. The relevant lambda or function could be changed to something that produces complete nonsense and these tests would never catch it. It's not really related to this PR though so I'd suggest opening a new issue.

Originally posted by @SGallagherMet in https://github.com/MetOffice/CSET/pull/765#discussion_r1740967262

We should make the tests in test_constraints.py more robust. This will likely involve making most of them into filter tests.